In electrically operated automatic transmissions of layshaft design having hydraulically controlled switching elements, the hydraulic system is controlled by means of electric energy. But if for any reason, such as on account of a short circuit, the gear control system no longer is supplied with electric energy, operation will no longer be possible in transmissions of the above mentioned kind having a gear control system known from the practice, and this necessarily results in that a vehicle, that is equipped with such a transmission, will be unfit to move and will break down.
It is true that from the practice reserve devices are known in planetary transmissions which, in case of electric failure, make possible a gearshift by a linking of numerous valves in the transmission, but such a reserve device is very expensive and cannot be transferred to a transmission of layshaft design.
In an electrically operated automatic transmission, a failure of electricity, for example, due to a short circuit in the bypass cables, can also result in, due to the unintentional engagement of a valve caused by the short circuit, two switching elements to work against each other whereby, the transmission becomes blocked. At the same time, it can result in instabilities in the behavior of the vehicle and create a situation critical for safety in which a vehicle, for example, can skid of the road.
From the prior art, safety devices which prevent a blocking of the transmission, as consequence of a short circuit, are known only in planetary transmissions, and such safety devices are impossible to use in transmissions of layshaft design, such as used in autobuses and delivery vans.
In addition, the reserve and safety devices, which in the practice can be used only in planetary transmissions, have the disadvantage that, without considering the situation of the vehicle in which the failure of electricity occurs, when an emergency gear is engaged which often is too low for the actual velocity of the vehicle and thus, overspeeding of the engine can occur.